The invention relates to the field of surface treatment and in particular to hydrophobic thermoplastic films for surface coatings and more particularly to films obtained by low-temperature drying of a latex containing neither volatile organic compounds nor coalescents.
In the context of the invention, the term xe2x80x9clatexxe2x80x9d should be understood to mean an aqueous dispersion of polymer particles which can be obtained by the emulsion polymerization of one or more monomers.
Many latices are intended for the building, paper, textile and leather industries and are used in the surface treatments of various materials, particularly as paints, varnishes, adhesives and mastics. More generally, such latices are required to have a minimum film-forming temperature, called hereafter MFT, as low as possible. The MFT is the lowest temperature at which the particles form a continuous film after the water has been evaporated.
When the polymers constituting the particles dispersed in a latex have a relatively low glass transition temperature, denoted by Tg, it is easy for the latex to form a film at room temperature but the films obtained are tacky and their mechanical integrity is poor. On the other hand, if the polymers have a high Tg, the films will be of better quality but film formation at room temperature would require the addition of a coalescents. The latter is generally a volatile organic compound which evaporates during application and film formation, as described in U.S. Pat. Nos. 4,455,402 and 5,021,469.
For obvious environmental-friendliness and cost-reduction reasons, it is desirable to develop latices containing neither volatile organic compounds nor coalescents, having a relatively low MFT and able to give films at low temperature which have no surface tack and are of good mechanical quality.
To achieve this objective, EP 466,409 proposes a solution based on a mixture of two latices, one having a low Tg, generally called a xe2x80x9csoftxe2x80x9d latex providing film formation and the other having a high Tg, generally called a xe2x80x9chardxe2x80x9d latex providing mechanical integrity. U.S. Pat. No. 263,193 describes a solution based on a shell/core latex with a shell having a xe2x80x9csoftxe2x80x9d character and a core having a xe2x80x9chardxe2x80x9d character. Unlike the previous document, EP 609,756 describes a latex, prepared in two steps, having a xe2x80x9csoftxe2x80x9d core with a Tg ranging from xe2x88x925xc2x0 C. to xe2x88x9255xc2x0 C. and a xe2x80x9chardxe2x80x9d shell with a Tg of less than 50xc2x0 C.
Likewise, FR 2,090,483 describes a latex, obtained by two-step polymerization, consisting of 35 to 50% of a polymer, obtained in the first step, having a Tg of less than 10xc2x0 C. and of 50 to 65% of a polymer, obtained in the second step, having a Tg of greater than 60xc2x0 C., the first-step polymer being crosslinked and fastened to the second.
Applicant has found that an ingenious solution to the problem described above entails a latex containing particles of hydrophobic polymers having a core/shell structure. With a so-termed xe2x80x9csoftxe2x80x9d core having a Tg of less than 20xc2x0 C. and a so-termed xe2x80x9chardxe2x80x9d shell having a Tg of greater than 50xc2x0 C. The Applicant has in fact discovered that, as long as the mass contribution of the shell to the total weight of the polymer particles does not exceed 30%, such a latex applied to any surface gives, after drying at low temperature, and without the use of coalescents or volatile organic compounds, a continuous film which has a structure in the form of soft parts dispersed in a hard matrix, is homogeneous, has no surface tack and has good mechanical integrity.
In general, hydrophobicity is the non-solubility in water or the lack of affinity with respect to water. According to the invention, this lack of affinity may be hierarchized. This is because the hydrophobicity according to the invention is defined by the solubility parameter; (delta) as described in xe2x80x9cProperties of polymersxe2x80x9d by D. W. Van Krevelen, 1990, 3rd edition, page 220. This parameter makes it possible to classify various polymers according to their affinities with respect to water. According to the invention, a polymer is hydrophobic if its (delta) is less than 26. In addition, if (delta1) of a polymer 1 is less than (delta2) of a polymer 2, then 1 is more hydrophobic than 2.
One of the subjects of the invention is a hydrophobic thermoplastic film consisting of 70 to 90% by weight of a polymer (P1) with a soft character having a Tg of less than 20xc2x0 C. dispersed in 10 to 30% of a polymer (P2) with a hard character having a Tg of greater than 50xc2x0 C.
The polymer P1 is dispersed in the form of individualized nodules in the matrix consisting of the polymer P2.
The film of the invention is hydrophobic in the context of the invention defined above; in addition, P1 is more hydrophobic than P2.
Preferably, P1 has a Tg of less than 0xc2x0 C. and P2 a Tg of greater than 60xc2x0 C.